Because with a turbo in a mode sequence, it is one more mode to have to cycle through and when you leave the turbo, you are back to the lowest mode if the sequence is from low to turbo.
With a turbo by double clicking, you can turn in ON when you are in any other mode and, depending on the UI, you can return to your previous mode when you leave the turbo mode. You can go to turbo from OFF by double clicking or you can go from OFF to a memorized mode by single clicking. This type of UI is more useful IMO.
If turbo has a step down, how do you reactivate turbo ? By cycling again throught all mode while it is much better by another double clicking.

And for those asking for 70.2 :

Habilite Official wrote:

MT03 is a flood light. We have no plan to upgrade it to XHP70.2. So MT03 will still have XHP70 version. and the beam color of MT03 XHP70 is nicer.